fix: diagnose unknown generic #builtin instead of silently returning 0 (issue 0144)

A bodiless #builtin with a $T: Type param routes through monomorphization.
When resolveBuiltin returned null for an unrecognized name, the builtin-body
branch fell through to ensureTerminator's constInt(0) -- a silent-fallback
default the CLAUDE.md REJECTED PATTERNS forbid. Emit a loud
'error: unknown #builtin <name>' diagnostic instead.

Regression: examples/1189-diagnostics-unknown-builtin.sx

examples/1189-diagnostics-unknown-builtin.sx

@@ -0,0 +1,16 @@
+// A bodiless `#builtin` carrying a `$T: Type` parameter, whose name the
+// compiler does not recognize, must fail LOUDLY with an "unknown #builtin"
+// diagnostic — not silently evaluate to 0 (the CLAUDE.md silent-fallback
+// pattern). The generic monomorphization path (monomorphizeFunction's
+// builtin-body branch) now diagnoses an unresolved builtin name instead of
+// falling through to `ensureTerminator`'s `constInt(0)`.
+//
+// Regression (issue 0144).
+#import "modules/std.sx";
+
+// `mystery` is not a recognized builtin.
+mystery :: ($T: Type, x: T) -> T #builtin;
+
+main :: () {
+    print("mystery(42) = {}\n", mystery(i64, 42));
+}

examples/expected/1189-diagnostics-unknown-builtin.exit

@@ -0,0 +1 @@
+1

examples/expected/1189-diagnostics-unknown-builtin.stderr

@@ -0,0 +1,5 @@
+error: unknown #builtin 'mystery'
+  --> examples/1189-diagnostics-unknown-builtin.sx:12:1
+   |
+12 | mystery :: ($T: Type, x: T) -> T #builtin;
+   | ^^^^^^^

examples/expected/1189-diagnostics-unknown-builtin.stdout

@@ -0,0 +1 @@
+